Speculation is rising that Vice President Joe Biden is close to a decision on entering the 2016 US presidential race even as the White House has been silent and President Barack Obama evaded queries on the issue.

The immediate provocation appeared to be an email written by Biden's close aide Ted Kaufman to his supporters that the vice president's campaign would be "optimistic", "from the heart" and unscripted.

The email was written late on Thursday and yesterday the International Association of Fire Fighters informed Biden that its union will endorse him if he enters the race. 

"If he runs, he will run because of his burning conviction that we need to fundamentally change the balance in our economy and the political structure to restore the ability of the middle class to get ahead," Kaufman wrote according to multiple media reports.

According to the Wall Street Journal, Biden is expected to announce in the coming days whether he will enter the presidential race and, at this point, signs point to him running for the Democratic nomination, people familiar with the matter said.